Quality Control in Pretreatment Process of Fabric

Standard test methods for the evaluation of individual processes in

 fabric preparation

Process	Test to be carried out
Desizing	Desizing efficiency
Scouring	Absorbency, wax content, ash content, cuprammonium fluidity
Bleaching	Absorbency, wax content, ash content, cuprammonium fluidity, whiteness
Mercerising	Barium activity number, other tests
Heat setting	Boiling water shrinkage, iodine absorption test

Desizing efficiency

ð  For the determination of desizing efficiency, the amount of size on grey fabric and residual size on the desized fabric is determined.

ð  According to the Indian Standard No. 199 the samples are first to be solvent extracted in a soxhlet with chloroform followed by enzyme desizing so that the size is completely removed.

ð  The desizing efficiency is then calculated as follows:

·         A desizing efficiency value of 90% is considered to be excellent and that of 80-90% is considered to be satisfactory. Values below 80% indicate poor desizing.

ð  This test procedure is rather cumbersome for routine work.

ð  Hence, the amount of size and residual size on grey and desized fabric may be determined by enzyme-desize only, using a good quality enzyme.

ð  Complete size removal may be checked by testing with acidified iodine solution.

% Desizing efficiency =  % Original size - % Residual size    X 100%

                                                                      % Original size

Absorbency

ð The simple test for measuring the absorbency of sample consists of allowing a drop of water to fall from a fixed distance (2.5 cm) to the conditioned fabric sample, which is mounted in an embroidery frame of about 6 inch diameter.

ð A stop watch is started as soon as the drop falls on the fabric and stopped no sooner the image of the reflected light disappears at the edge of the drop. i.e. water drop is completely absorbed by the fabric.

ð This is termed as “Drop Absorbency Test”.

ð Another method for absorbency test, is the measurement of the time required for the sample of about 1 inch size to sink in water, termed as Sinking Time.

ð Drop absorbency or sinking time of about 5 sec is generally considered satisfactory for well prepared cellulosic materials.

ð In the case of polyester which is a hydrophobic fibre, or its blends, the above mentioned test methods are not likely to be applicable.

ð Berdnt and Golob have suggested measuring the height of water raised by capillary action to indicate the uniformity in absorbency of polyester/cellulosic blended fabric.

ð In this test method a 5 cm wide strip is cut across the filling direction. The strip is then cut into 5 cm long sections.

ð The numbered specimens are then immersed 1 mm deep in 1% aqueous solution of C.I.Direct Blue 86 for 2 sec and then immediately placed on a wire screen.

ð After drying, the capillary rise of the dye solution is measured. The uniformity in absorbency across the width can thus be tested.

ð For 100% polyester fabric a test method has been suggested by Dugal et. Al.

ð Specimens measuring 50 cm long(warp) X 30 cm wide(filling) are  cut from the fabric, across the full width.

ð These are then padded with 60 gpl black disperse dyestuff and 10 gpl polyacrylate based antimigrating agent, with a pick-up of about 70% and finally air dried.

ð The differential absorbency, if any, can be seen on the dried ground in the form of light-dark stripes, spots or streakiness.

ð In many cases the fault can be detected in the padded fabric itself.

Wax Content

ð  This is estimated by extracting about 10 g of material with a suitable solvent (viz Chloroform for cellulose and petroleum ether for polyester blends) in a soxhlet apparatus for about 4hours.

ð  The extracted is filtered and the filtrate is dried on a water bath and then transferred to an oven, kept at 105°C and dried to constant weight.

ð  The amount of residue is expressed as percentage wax content on the original weight of the sample.

Ash Content

ð  About 5 g of sample is incinerated in a silica crucible in a burner followed by complete ashing in a muffle furnace at about 700°C for 10 min and the amount of ash obtained is expressed as percentage of the original sample.

Cuprammonium Fluidity

ð  This is a sensitive index for the determination of chemical degradation of cotton cellulose during scouring, bleaching, souring, etc.

ð  In this test, the conditioned cotton sample required to make 0.5% solution is exactly weighed and dissolved in cuprammonium hydroxide solution in an X- type viscometer tube and the rate of flow of this solution at temperature of 20°C.

ð  The viscometer is a glass tube with a wide mouth at one end and a narrow capillary exit, through which the solution is allowed to flow, at the other end.

ð  The dimension of the tube should be within the limits specified in the various standards.

ð  As the dimensions of individual viscometer tubes will vary within the prescribed limits, certain preliminary determinations have to be made with each tube, which should preferably by numbered suitably for identification.

ð  Firstly, as a 0.5 % solution is to be prepared in each tube, the volume of each tube has to be determined accurately.

ð  This internal volume should be approximately 20 ml and is determined by completely filling the viscometer tube with water, with a mercury column of 0.7 ml in position at the bottom, as this quantity of mercury is always added in each viscometer for the purpose of stirring the solution.

ð  Secondly, the constant (C) of each viscometer has to be determined by measuring the time of flow (t₁) in seconds of a liquid of known fluidity; for this purpose, a solution of about 65% glycerol in water, having a density (d₁) of 1.681 g/cm³ and a fluidity (F₁) of 6.83 rhes is used.

ð  Then,

C  =  F₁  X  d₁  X  t₁

ð  The flow time of this solution between two fixed marks on a calibrated viscometer (fluidity tube) is measured at a specific temperature.

ð  The fluidity value “F” is then calculated from the equation given below

F = C / t ,

C = Viscometer constant and t = Flow time

ð  The results are expressed as rhes (1/Poise), which is the reciprocal of the unit of viscosity.

ð  Fluidity value 5 to 8 is considered to be satisfactory for normal bleached fabric.

ð  Determination of fluidity is a sensitive test for fabrics of 100% cotton; but erratic results may be obtained when cotton is blended with regenerated cellulosic which have a much lower DP than cotton and are therefore prone to suffer damage during alkaline scouring.

ð  However, for fluidity determination of 100% viscose fabric, a 2% solution in cuprammonium hydroxide may be used which is likely to show a fluidity value of 8.

Whiteness

ð  This is attributed to the luminosity as well as to freedom from yellowness.

ð  It is measured by measuring reflectance of the specimen against a standard white, viz. magnesium oxide which represents a whiteness value of 100.

ð  Whiteness is measured at 460 nm, measures both the brightness and yellowness of the sample since this wavelength lies in the blue region of the spectrum.

ð  This measurement at single wavelength is suitable only for comparing samples having the same reflectance curve and often do not agree with visual assessment.

ð  Therefore whiteness formulae, as given below, based on reflectance measurements at two different wavelengths i.e. blue and red region of the spectrum, have been developed.

ð  This means that both the general level of reflectance and the reflectance loss in the blue region i.e. degree of yellowing are recorded.

W = 100 – (R670 – R430)  ……. (Harrison)

W = 430 – (R670 – R430) ……. (Stephenson)

ð  The expression within the bracket is a direct measure of the degree of yellowing.

ð  In the Stephenson formula the reflectance measured in the blue region also serves as an intrinsic white, whereas in the Harrison formula the constant 100 is used and intrinsic white is not included.

ð  Development of tristimulus spectrophotometer and calculation of whiteness value using B.G.A. system (B = blue, G = green, A = amber i.e. red) have significantly improved the results and these are claimed to tally with visual assessment.

ð  In the USA, many instruments in common use give the Hunter’s coordinates L, a, b directly and whiteness formulae based on these values have been developed.

ð  Two such important formulae are given below:-

W = L – 3b      ….. (Hunter)

W = L + 3a – 3b … (Stensby)

Barium Activity Number

ð  The degree of swelling of cotton material during mercerisation is usually determined by the barium hydroxide absorption test.

ð  Mercerised cotton absorbs soluble alkaline hydroxide to a greater extent than unmercerised cotton.

ð  Amongst the various alkalies, barium hydroxide is more readily absorbed than the others, and the extent of absorption can also be more easily measured.

ð  The barium activity number is defined as the ratio of the quantity of barium hydroxide absorbed by mercerised cotton to that absorbed by unmercerised cotton under identical conditions, multiplied by 100.

ð  A specimen weighing about 3 g is taken from the sample and is extracted with carbon tetrachloride in a soxhlet extraction apparatus to remove minerals, waxes etc.

ð  Exactly 2 g (oven – dry weight basis) of the specimen is transferred to a 50 ml conical flask and 30 ml of 0.25 N barium hydroxide added to it.

ð  The flak is stoppered and allowed to stand for at least 2 hours with frequent shaking.

ð  At the end of the period, 10 ml of the clear solution are pipette out from the flask and titrated with 0.1 N hydrochloric acid using phenolphthalein indicator.

ð  An unmercerised scoured cotton material similar in construction to that under test is used as a control and the determination is carried out on the control specimen in the same manner as on the mercerised material.

ð  A blank determination is also carried out following the same procedure but without any specimen in the flask.    

ð  From the results, the barium activity number is then calculated as follows:-

Barium Activity Number =  a – b  X  100

                                                 a – c

   

 Where, a = volume of acid required for blank

                          b = volume of acid required for mercerised test specimen

                          c = volume of acid required for control unmercerised specimen

ð  A well mercerised cotton fabric generally shows barium activity number in the range of 125 – 135.

ð  When cotton is blended with polyester fibre this test may be used for routine checking.

ð  In such cases the amount of sample should be adjusted (depending on blend composition) so that the amount of cotton fibre in the blended sample equals to 2 g.

ð  However, it should be clearly noted that this test may be followed only for a routine check and these results cannot be referred to for any claim or litigation as no standard has yet been published on this subject.

ð  When cotton is blended with regenerated cellulosics, the determination of barium activity number is not likely to give correct results as regenerated cellulosics have a different absorption of barium hydroxide as compared to cotton.

Shrinkage in Boiling Water

ð  This test was originally recommended by Du Pont, for assessing the efficiency of heat setting of polyester/cotton blended fabric.

ð  In this test, the blended fabric sample is marked in warp and weft directions and is then boiled in water in drum washing machine for 30 minute.

ð  The boiling water shrinkage is then determined.

ð  It has been found that a well heat set, 67/33, polyester/cotton blended fabric shows boiling water shrinkage within 1%.

ð  However, this may very if the proportion of cellulosic component in the blend is increased and standards for different blends can easily be established.

ð  It is calculated as follows:-

% Shrinkage =  L₀  -  L_f     X   100

                                             L₀

                Where, L₀ = Original Length

                         L_f  = Final Length

Iodine Absorption Test

ð  Heat set polyester absorbs less iodine than the corresponding unheated material and this property is used for assessing the degree of heat setting of polyester.

ð  Exactly 1 g of sample is accurately weighed into 250 ml stoppered flask and 30 ml of 0.1 N iodine solution (prepared by dissolving 12.7 g iodine and 20 g potassium iodide in water. To this solution 100 ml glacial acetic acid and 350 ml phenol is added and finally diluted to 1 : 1 with water) is added and allowed to stand for 2 hours.

ð  After this period, the specimen is transferred to a sintered glass crucible and washes with water till free from iodine.

ð  The sample is then transferred to a 250 ml flask containing 50 ml chloroform.

ð  As the chloroform is a powerful swelling agent for polyester, the iodine absorbed by the polyester quickly passes from the fibre to the chloroform.

ð  Exactly 10 ml of 0.1 N sodium thiosulphate is added and the mixture is titrated against 0.01 N iodine solution, using starch as indicator.

ð  Simultaneously a blank determination (without sample) is carried out under the same conditions.

ð  The absorption of iodine is expressed in mg of iodine per g of fibre i.e.

Iodine Absorption (mg/g) =   (X – Y) X 0.01 X 127

                                                                            W

Where, X = ml of 0.01 N iodine required for blank

             Y = ml of 0.01 N iodine required for sample

                        W = weight in g of sample

ð  If polyester is blended cellulosic fibre, the latter should be removed by carbonisation and only the polyester portion should be taken for the test.

ð  It should be noted that the test temperature has a significant effect on iodine absorption by polyester.

ð  With increase in test temperature the iodine absorption value also increases.

ð  It is therefore necessary to carry out the tests at the same temperature, in order to obtain reproducible results.

ð  This test is also carried out for cotton material to know accessible region of it.

ð  An accurately weighed quantity, about 0.3 g of the sample of cotton material is treated in a 250 ml conical flask with 2 ml of iodine solution (containing 5 g iodine and 40 g potassium iodide, dissolved in 50 ml distilled water).

ð  After thoroughly mixing the cotton and iodine solution, 100 ml of a saturated solution of sodium sulphate are added to the flask.

ð  The flask and the contents are stored in a dark place for about one hour. The unabsorbed iodine remaining in the solution is determined by titrating 50 ml of the solution, to which are added 50 ml of distilled water, with 0.02 N sodium thiosulphate solution.

ð  Starch solution (1 ml of 1% solution) is used as the indicator. A blank determination on the original iodine solution is carried out in the same way without any specimen in the flask.

ð  From the titration readings, the percentage of accessible region is calculated by the formula:

Percentage accessible region  =  (a-b)  X  2.04  X  2.54     X    100

                                                                             412  X  c

            Where, a = quantity in ml of sodium thiosulphate solution required by the blank

                         b = quantity in ml of thiosulphate solution required by the solution containing

                             the test specimen

                         c = oven – dry weight of the specimen taken for the test.

Copper Number                                

ð  The presence of aldehydic groups in the cellulose molecular chain can be assessed from its reducing power determined as the copper number.

ð  The ‘copper number’ is the weight in grams of copper reduced from the cupric to the cuprous state in alkaline solution by 100 g of oven – dry cellulose.

ð  The value of copper number is very low (about 0.2) in the case of normal cotton fibres and there is a marked increase in the copper number when chemical changes are introduced in the molecules, by oxidizing agents, acids or alkalies.

ð  The two methods commonly used for the estimation of copper number are

(1)   the Schwalbe – Braidy method, as modified by Clibbens and Geake

(2)   the ‘micro’ method developed by Heyes.

ð  The second method is based on the use of small quantities of the material and is widely used for routine determinations and for research work.

ð  For ‘micro’ method following solutions are first prepared:-

·  Solution A:- Prepared by dissolving 150 g anhydrous sodium carbonate and 50 g sodium bicarbonate in one litre of water.

·  Solution B:- Prepared by dissolving 100 g crystalline copper sulphate in one litre of water.

·  Solution C:- Prepared by dissolving 100 g ferric alum in water, adding 140 ml concentrated sulphuric acid and then making up the volume to 1 litre with water.  

ð  The sample is cut into pieces, approximately 1.5 mm in length, and then thoroughly mixed.

ð  An accurate weighed 0.25 g of the sample (oven – dry) is taken in a Pyrex test tube (100 mm X 15 mm).

ð  A mixture of 9.5 ml of solution A and 0.5 ml of solution B is heated quickly to boiling and poured over the weighed sample in the test tube.

ð  The test tube is covered with a pear – shaped bulb to prevent evaporation and then immersed in water bath containing boiling water such that the level of the liquid in the test tube is below that of the water in the bath.

ð  After the boiling has proceeded for 10 min the contents of the test tube are stirred with a glass rod to drive off carbon dioxide.

ð  The boiling is continued for three hours, with stirring being carried out at regular intervals of 30 min.

ð  The tube is then removed and cooled in water. The contents of the tube are filtered using a sintered crucible (IG3) and the residue washed thrice with water.

ð  The crucible is then attached to a clean filter flask and the cuprous oxide residue in it is dissolved by treatment with solution C.

ð  For this purpose, the reaction tube is rinsed with 1.5 ml of solution C until all the oxide deposited on the inside surface of the tube is dissolved.

ð  Without applying any suction to the filter flask, the residue in the crucible is flooded with this solution from the tube, and the solution is allowed to remain in contact with the precipitates for about two minutes.

ð  Suction is then applies to the flask and the above treatment repeated by taking 1 ml of solution C in the reaction tube.

ð  The reaction tube is washed out in the filter and the cellulose residue in the crucible washed three to four times, using 2 ml lots of distilled water and squeezing the cellulose with a glass rod after each washing.

ð  The filtrate and washing are titrated with 0.04 N ceric sulphate, using ferrous – o – phenanthroline as internal indicator.

ð  A blank determination is carried out using 2.5 ml of solution C and the same quantity of distilled water as used for washing.

ð  It may be noted that 1 ml of 0.04 N ceric sulphate is equivalent to 0.002543 g copper.

ð  The copper number is then calculated from the volume of ceric sulphate required and the quantity of oven – dry cellulose taken up for the test.

ð  In the case of copper number exceeds 4, the determination is repeated with half the weight of the sample.

ð  Duplicate determinations are carried out in each and the mean is calculated.